Laser beam power measurement

ABSTRACT

Absolute measurement of the power output of a radiation energy utilizing the intensity of fluorescence of the secondary emission process from a body of a molecular crystal irradiated by the radiation energy under measurement.

United States Patent Toriyama [4 1 Apr. 25, 1972 s41 LASER BEAM POWERMEASUREMENT 3,070,698 12/1962 Bloembergen ..2s0/s3.3 IR

[72] Inventor: Kazuhisa Torlyama, Tokyo, Japan OTHER PUBLICATIONSAssignee! Himchi, -v y Japan Triplet Excitons and Delayed Fluorescencein Anthracene 22 F] d: S L 18 1969 Crystals by Kepler et a]. PhysicalReview Letters (10) No. 9 1 ep Mayl63pp.40002 [21] Appl. No.: 858,980

Primary Examiner-James W. Lawrence Assistant Examiner-D. C. Nelms [52]US. Cl. ..250/71.5, 250/833 R Attorney Craig Antonem & i [51] Int. Cl...G0lt 39/18 [58] Field ofSearch ..235/l5l.3, 193; 250/715 R, [57]ABSTRACT 250/71 R, 83.3 IR, 83.3 R; 252/3012, 301.3

Absolute measurement of the power output at :1 radiation {56] ReferencesCited energy utilizing the intensity of fluorescence of the secondaryemission process from a body ofa molecular crystal irradiated UNITEDSTATES AT by the radiation energy under measurement.

2,974,231 3/1961 Greenblatt et al. ..250/83.3 4 Claims, 6 DrawingFigures ANTHR JCE/VE RUB) P00 v l /0 SPECFPOSCOPE :1

/2 AMPL/FVER 4\cH/?05m95- PATENTEDAPR 25 I872 SHEET 2 [IF 4 77/145 secHa s BACKGROUND OF THE INVENTION 1. Field of the Invention:

This invention relates to an absolute measurement of the power of alaser beam and more particularly to a method and an apparatus formeasuring laser beam power utilizing the fluorescence of a molecularcrystal emitted in the secondary process.

2. Description of the Prior Art Lasers have wide and various potentialsin their application based on their monochromaticity, high intensity andcoherency. conventionally, the power output of a laser beam has usuallybeen measured by a photoelectric or calorimetry system.

However, regarding photoelectric measurement, the sensitivity of aphotoelectric cell depends on the wavelength of the light beam undermeasurement so that precise calibration is needed before an absolutemeasurement can be taken. In a thermal detector (for example, athermocouple), the heat capacity is rather large for light beamdetection, so that the time constant for measurement is too large toprecisely detect a rapid laser beam radiation such as a ruby laser or agiant pulse.

SUMMARY OF THE INVENTION An object of the present invention is toutilize in instruments for laser beam power detection the fluorescenceof a molecular crystal emitted in the secondary process so as to providea rapid response beam power measurement by which absolute measurement isenhanced and which has eliminated the above drawbacks of theconventional beam power meters.

Another object of the present invention is to provide a direct readingbeam power meter comprising means for reading out the power of theinitial light beam irradiated onto molecular crystals utilizing thefluorescence emitted in the secondary process of the molecular crystal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an energy diagram of amolecular crystal, anthracene, for use in the beam power measurement ofthe present invention;

FIG. 2 is a block diagram of a beam power measuring system ofthe presentinvention;

FIG. 3 shows the intensity of fluorescence characteristics of the firstand the second processes of anthracene vs. time;

FIG. 4 is a photograph of fluorescence obtained on the face ofasynchroscope by the measuring system of FIG. 2;

FIG. 5 is a correlation plot diagram of the inverse square root of theintensity of fluorescence obtained by the light beam power measuringsystem of the present invention vs.

5 time; and

FIG. 6 is a block diagram of a direct reading beam power measuringsystem of the invention having an electronic computing circuit fordirect reading.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As is shown in FIG. 1, theenergy levels of a molecular crystal of anthracene comprise a groundstate 1 A,), an excited state 4 A,, B another excited state 2 (Bintermediate between the states 1 (A,,) and 4 (A, B,), and yet anotherexcited state 3 (B intermediate between the states 4 (A B,,) and 2 (BWhen a laser beam 7 (6943 A) is emitted from a ruby rod 6 onto a body ofanthracene crystal 5 as is shown in FIG. 2, the anthracene molecules inthe ground state 1 (A are elevated to the excited state 4 A, B,,) by thetwo photon absorption of the laser beam and then shift down to theexcited state 3 8 by a non-radiative transition. From the state 3 B somemolecules directly return to the ground state 1 (A,) by the firstprocess fluorescence and the rest shift down to the state 2 (B bynon-radiative transition. The initial concentration y of such moleculesin the excited state 2 (B can be represented as y..= sT 1 1) where:

K is the rate constant of radiationless singlet-triplet conversion fromexcited states 3 B to 2 (B i.e. the proportional constant representingthe transition probability from the state 3 to the state 2,

t, is the lifetime for the excited state 3 8 t is the duration ofirradiation of the laser beam,

6 is the coefficient of two photon absorption and is thus the absorptioncoefficient in the case of one photon absorption, and

I is the intensity of the irradiating laser beam. Here, the molecularconcentration of the excited state 2 (B in an equilibrium state beforethe irradiation by a laser beam is neglected. I

Anthracene molecules in the excited state 2 (B1) having the aboveconcentration shift up to the state 3 UB by collisions of a tripletexciton and then return to the ground state I ('A,,) with emission offluorescence of the secondary process.

Setting the concentration of anthracene in the state 2 (B in the aboveprocess as y, the variation of concentration per unit time is written asy/ w (2) Here, the transition probability from the state 2 (B to theground state 1 A,) is neglected since it is a forbidden transition andthe time needed for a molecule in the state 2 to exhaust its tripletexciton energy and transit to the ground state is far longer than thatfor the transition from states 2 (B to 3 8 with bimolecular reaction.

Solving differential equation (2) with condition of equation l theconcentration of anthracene becomes yo) y /(vyn (3)- In the secondaryprocess, such anthracene molecules get energy to transit to the state 3and then to the ground state by the emission of fluorescence. Theintensity F(t) of such fluorescence is 0) v [y /(Wo 4). This relationcan be seen in FIG. 3 in which the abscissa represents time (second) andthe ordinate represents the relative intensity. Curve 8 represents thefluorescence emitted in the first process and curve 9 that for thesecondary process.

From equation (4), it is seen that the inverse square root of v theintensity 1/ /1 and time are in a linear relation of l ri I v 1m 11 1/0The line has a slope of (Z'y/A)" and an expected extrapolated value of/2 Av)" yol' at t 0 (which is not the time of irradiation but a certaintime after the irradiation). Dividing the slope by the extrapolatedvalue,

The present invention is based on the above theoretical analysis on thefluorescence of the secondary process of a molecular crystal. Now,embodiments of the invention will be described hereinafter.

EMBODIMENT 1 from the anthracene body is detected with a photomultiplier12 through a spectroscope or a CuSO solution filter 11. Thephoto-current obtained from the photomultiplier 12 is supplied to asynchroscope 14 through an amplifier 13 to give the curve offluorescence,such as curves and 15 in FIG. 4. In the figure, theordinate represents the intensity of fluorescence while the abscissarepresents time- Tail portions 15 and 15' of the fluorescence curvecorrespond to the fluorescence in the secondary process. The result ofanalysis of FIG. 4 is shown in the l/ F (1) vs. time (micro-second)plotting of FIG. 5 where l/ I F (t) is in an arbitrary unit. Theconstants used in this embodiment were as follows:

K X 10 sec t 3 X10 sec y= 5 X10 sec. Thus, I 2 X 10 photon.cm .sec isobtained.

EMBODIMENT 2 A laser beam 7 is directed from a ruby rod 6 onto amolecular crystal of anthracene 5 as is shown in FIG. 6. Fluorescence 10emitted from the anthracene 5 is detected by a photomultiplier 12through a spectroscope (or CuSO, filter) 11.

The photo-current of the photomultiplier 12 which becomes proportionalto the intensity of fluorescence F after a certain time has passed fromthe initial irradiation obeys theoretical equations (5) and (6) as isdescribed above. FIG. 6 shows an embodiment of an electronic computingcircuit for calculating the beam power 1,, of a laser beam. In thefigure, a photo-current F is supplied to a terminal A from which it istransferred through an amplifier 16 to both a differentiation circuit 24and a gate circuit 17. The differentiation circuit 24 shapes theadmitted current signal into a pulse and sends it to gate circuits 17and 27 through delay circuits and 26 to open the gates. Upon releasingthe gate circuit 17, the photocurrent F is admitted through the gate toa function generator 18 which generates the square root of an input. Thedelay time of the delay circuit 25 is preferably the duration of thefirst process fluorescence i.e. about 3X 10- sec. The square root \/I-is supplied from the function generator 18 to an inverse numbergenerator 19 to obtain I/ F which is sent on one hand to adifferentiation circuit 20 to become 0(1/ F )/in and on the other handto an initial value hold-up circuit 28 upon the release of the gatecircuit 27. The inverse of the square root 1] J F t=t fromthe hold-upcircuit 28 and the differential of the inverse square root 6( l/ V F(t))/8t are multiplied by My at a multiplier circuit 21 I is the delaytime of the delay circuit 26 relative to the rise of the first processfluorescence) and sent to another multiplier circuit 22 to perform amultiplication of The square root of this value is obtained at afunction generator 23. The beam power of the laser beam 7 1. it does notmeasure the beam power of a laser beam itself but the fluorescence of amolecular crystal due to the irradiation by a laser beam so thatabsolute measurement of the beam power is possible with a photoelectricdetector of high sensitivity (one-, two-, or three-photon absorption ormore can be utilized);

2. further, in absolute measurement, the beam power can be measured bydetecting only a part of the fluorescence from the crystal bytransmitting a laser beam through a crystal; and

3. there is no need to reduce the laser beam power with a filter as isthe case with conventional photoelectric detection causing no problem ofthe need for calibrating a filter and there is also no need to calibratea detector.

It is to be noted that other molecular crystals similar to anthracene,such as phenanthrene, phyrene, or 3,4-benzpyrene, which emitfluorescence in the secondary process, can also be utilized in theinvention.

1 claim:

1. A method of measuring a beam power comprising the steps of directinga laser beam to be measured onto a body of molecular crystalwhich canemit fluorescence of the secondary process, a second step of measuringthe intensity of fluorescence of the secondary process emitted from saidmolecular crystal upon irradiation, and a third step of calculating theabsolute value of the beam power to be measured from said intensity offluorescence.

2. A method of measuring a beam power according to claim 1, wherein thesecond step is done with a photoelectric detector after the lifetime ofa singlet excited state has passed after the irradiation, and the thirdstep comprises amplifying the output photo-current of said detector,taking the square root thereof, and the inverse square root, holding upthe inverse of the square root for a certain time on one hand,differentiating the inverse of the square root on the other hand,multiplying both of these values and the inverse of the product of thelifetime of a singlet state of the crystal, the duration of irradiation,coefficient of photon absorption for the transition to said singletexcited state, and the triplet-triplet annihilation rate constant, andtaking the square root of their product to obtain the beam power.

3. A beam power meter comprising:

a body of a molecular crystal for receiving and transmitting a laserbeam to be measured, the molecular crystal being capable of emittingfluorescence of the first and secondary process;

a photoelectric detector means for detecting the intensity of thefluorescence from said body after the lifetime of the singlet excitationstate has passed from the time of the emission of fluorescence;

amplifier means for amplifying the photo-current F from said detectormeans;

a first circuit means including a differentiation circuit for shapingthe photo-current into a pulse, a delay circuit for giving a delay tothe pulse signal, and a gate circuit for receiving the delayed pulsesignal;

a second circuit means including a gate circuit for receiving thephoto-current F, a delay circuit for receiving said shaped pulse signaland sending it to the gate circuit with a delay to open the gate, afunction generator for taking the square root of the photo-current F,and an inverse number generator for taking the inverse of the squareroot of the photo-current l/ F T) and sending it to both the gatecircuit of said first circuit means and a differentiation circuit;

a hold-up circuit for receiving the initial value of l/ F through thegate circuit of the first circuit means upon releasing of the gate;

multiplier circuit means for receiving 1/ VF from the hold-up circuitand l 1/ V F (z) 1 K being the rate constant of radiationlesssinglet-triplet T T conversion from the excited state B to the excitedstate I i being the lifetime of the excited state B and a functiongenerator connected to the multiplier circuit 5 n being the durationCHI-radiation f the laser beam under for taking the square root of the.input measurement and r- 6 being the coefficient of two photonabsorption. 4. A beam power meter according to claim 3, wherein said VF1=1l 7 J l0 molecular crystal is selected from the group consisting ofto obtain the beam power of the laser beam to be measured; anthracenepyrene phenamhrene and 34'benzpyrene 7 being the triplet-tripletannihilation rate constant,

UNITED STATES PATENT OFFTCE CERTIFICATE OF CORRECTION 5 Patent No. 3,659, 102 Dated April 25, 1972 Inventor(s) Kazuhisa Toriyama It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Page 1, left-hand column between item [21] and item [52] insert 30]foreign application priority data September 20, 1968, Japan, 67630/68Signed andsealed this 9th day of January 1973.

(SEAL) Attest: v

EDWARD M.FLETCHER,JR. ROBERT GOTTSEHALK Attesting Officer Commissionerof Patents FORM PO-105O (10-69) USCOMM-DC 6O376-P69 u.s. GOVERNMENTPRINTING OFFICE: 1969 o-sss-3a4

1. A method of measuring a beam power comprising the steps of directinga laser beam to be measured onto a body of molecular crystal which canemit fluorescence of the secondary process, a second step of measuringthe intensity of fluorescence of the secondary process emitted from saidmolecular crystal upon irradiation, and a third step of calculating theabsolute value of the beam power to be measured from said intensity offluorescence.
 2. A method of measuring a beam power according to claim1, wherein the second step is done with a photoelectric detector afterthe lifetime of a singlet excited state has passed after theirradiation, and the third step comprises amplifying the outputphoto-current of said detector, taking the square root thereof, and theinverse square root, holding up the inverse of the square root for acertain time on one hand, differentiating the inverse of the square rooton the other hand, multiplying both of these values and the inverse ofthe product of the lifetime of a singlet state of the crystal, theduration of irradiation, coefficient of photon absorption for thetransition to said singlet excited state, and the triplet-tripletannihilation rate constant, and taking the square root of their productto obtain the beam power.
 3. A beam power meter comprising: a body of amolecular crystal for receiving and transmitting a laser beam to bemeasured, the molecular crystal being capable of emitting fluorescenceof the first and secondary process; a photoelectric detector means fordetecting the intensity of the fluorescence from said body after thelifetime of the singlet excitation state has passed from the time of theemission of fluorescence; amplifier means for amplifying thephoto-current F from said detector means; a first circuit meansincluding a differentiation circuit for shaping the photo-current into apulse, a delay circuit for giving a delay to the pulse signal, and agate circuit for receiving the delayed pulse signal; a second circuitmeans including a gate circuit for receiving the photo-current F, adelay circuit for receiving said shaped pulse signal and sending it tothe gate circuit with a delay to open the gate, a function generator fortaking the square root of the photo-current Square Root F, and aninverse number generator for taking the inverse of the square root ofthe photo-current 1/ Square Root F (t) and sending it to both the gatecircuit of saId first circuit means and a differentiation circuit; ahold-up circuit for receiving the initial value of 1/ Square Root Fthrough the gate circuit of the first circuit means upon releasing ofthe gate; multiplier circuit means for receiving 1/ Square Root F t fromthe hold-up circuit and from the differentiation circuit of said secondcircuit means calculating and a function generator connected to themultiplier circuit for taking the square root of the input to obtain thebeam power of the laser beam to be measured; gamma being thetriplet-triplet annihilation rate constant, KST being the rate constantof radiationless singlet-triplet conversion from the excited state 1B2uto the excited state 3B2u, tf being the lifetime of the excited state1B2u, t11 being the duration of irradiation of the laser beam undermeasurement, and delta being the coefficient of two photon absorption.4. A beam power meter according to claim 3, wherein said molecularcrystal is selected from the group consisting of anthracene, pyrene,phenanthrene, and 3,4-benzpyrene.